Comparison of the in-plane shear response of a FRP obtained by biaxial tension-compression test and standard methods

Standard methods for determining the intralaminar shear behaviour of unidirectional fibre-reinforced polymers (FRPs) provide accurate measurements of shear stiffness along the principal material directions. However, the shear stress-strain relationship often varies across methodologies, leading to significant differences in the onset of non-linearity and the estimated shear strength. In this work, the Digital Image Correlation technique is used to assess the full strain fields obtained from V-notched rail, Iosipescu and uniaxial tensile standards, together with the tension-compression biaxial test (TC test), recently introduced as a novel technique for determining the in-plane shear response. The strain state specific to each case is examined, as they are closely related to the differences observed in their non-linear shear responses. This study presents, for the first time in the literature, a comprehensive comparison of angular changes, rigid-body motion and fibre reorientation obtained with different shear test methodologies. The main challenge lies in quantitatively describing the deviation from ideal strain states, i.e. simple and pure shear strain fields, as large shear strains during the non-linear shear response can significantly alter the initial geometry of the specimen. As a new outcome, shear and longitudinal strains along the updated fibre directions are calculated for analysing which strain component is predominant throughout the experiments. This comparative information is intended to support decision-making when selecting the most appropriate methodology. If only the shear modulus in the principal directions is of interest, the simpler and more cost-effective uniaxial tensile test is recommended first, followed by V-notched type methods. For a more detailed characterization of the non-linear shear behaviour, the additional time and expense of TC tests may be justified.